There does not seem to be an easy way to set your Thunderbird to use a persona, so until we make it more discoverable in the UI, here's a snippet you can copy and paste in your error console (Tools > Error Console) to make your Thunderbird use a nice Persona! Just copy the line below in Tools > Error Console, and hit Enter. This will open a new tab allowing you to navigate getpersonas.com.

top.opener.document.getElementById("tabmail").openTab("contentTab", { contentPage: "https://www.getpersonas.com" });